The present invention relates to an engine controller and a method for controlling an engine that performs fuel cut-off.
One example of a vehicle engine determines the fuel injection amount in accordance with the mass of air that exists in a cylinder (hereinafter referred to as “the in-cylinder air amount”) when combustion starts in the cylinder and controls the air-fuel ratio of an air-fuel mixture burned in the cylinder at a desired value. Some of the burned gas generated by the combustion in the cylinder is not scavenged and remains in the cylinder. That is, the residual burned gas exists in the cylinder during the next combustion. This is referred to as internal exhaust gas recirculation (internal EGR). Such residual burned gas is inert gas that does not contribute to combustion. Thus, when combustion is successively performed, the residual burned gas may be ignored, and the fuel injection amount may be determined taking into account only the amount of fresh air supplied to the cylinder in the intake stroke (hereinafter referred to as “the in-cylinder intake air amount”).
A vehicle engine may perform fuel cut-off that temporarily stops fuel injection such as when the vehicle is coasting to reduce fuel consumption. Combustion does not occur during the fuel cut-off, and the residual burned gas is replaced by air in the cylinders. The first time combustion is performed in each cylinder after the fuel cut-off ends and fuel injection restarts, and the amount of air existing in each cylinder corresponds to the total of the mass of air remaining in the cylinder from the previous cycle (hereinafter referred to as “the residual air amount”) and the in-cylinder intake air amount. Consequently, if the fuel injection amount is determined based on the in-cylinder intake air amount, the air-fuel ratio becomes leaner than expected. This may lead to misfiring or cause undesirable emission.
In Japanese Laid-Open Patent Publication No. 2006-329065, the residual air amount is obtained. Then, the residual air amount is added to the amount of fresh inflow air to obtain the in-cylinder air amount for the first combustion performed in each cylinder after fuel injection is restarted. The residual air amount is calculated based on the engine rotation speed, the intake pressure, and the valve overlap amount.
In the described technique, the in-cylinder air amount for the first combustion in each cylinder after fuel injection restarts takes into account the residual air amount. However, the burned gas generated before fuel cut-off starts remains in the cylinder immediately after the fuel cut-off starts. Such residual burned gas is not completely scavenged from the cylinder in the first exhaust stroke after the fuel cut-off starts. Some of the burned gas remains in the cylinder even after the first exhaust stroke. The replacement of residual burned gas with air during the fuel cut-off does not suddenly occur immediately after the fuel cut-off starts. The replacement occurs in a stepped manner whenever the intake-exhaust cycle is repeated. The described technique does not take into account such stepped scavenging of the residual burned gas that occurs during the fuel cut-off. Thus, if the fuel cut-off ends within a short period, the residual air amount may not be accurately obtained.
It is an object of the present invention to provide an engine controller and an engine control method that obtains the amount of air remaining in a cylinder from a previous cycle when restarting fuel injection and performing combustion for the first time.